In slide hopper curtain coating operations, the liquid to be coated onto a moving substrate issues from the hopper slot and flows down the slide to the hopper lip. As the liquid exits the hopper lip, there is formed a generally planar liquid sheet or curtain which falls freely by the action of gravity. This resulting curtain is deposited on the moving substrate at a point of impingement or, more accurately, a line of impingement. The action of the moving web substrate or web induces a flow of air forming a boundary layer. The detrimental effect of boundary-layer air resulting from the movement of the web is known. Also known are methods for mitigating the detrimental effect of boundary-layer air. One approach was taught in U.S. Pat. No. 3,508,947 to Hughes in which the air entrained on the moving web is minimized by the use of an air shield that has been provided with a vacuum manifold which is positioned adjacent the web to be coated and connected to a vacuum pump to withdraw air therefrom. In this manner, Hughes proposes that the multi-layer, free-falling vertical curtain is shielded from ambient air currents and the air entrained by the moving web is drawn off before the curtain impinges on the moving web.
More recent curtain coating practice employs the air shield mainly for the purpose of drawing off air entrained by the moving web as opposed to shielding the free-falling curtain from ambient air currents. This is because curtain coating operations now typically include an enclosure to shield the free-falling liquid curtain from ambient air currents. The enclosure is continuously supplied with laminar low velocity air flow from the top while, at the same time, air is exhausted from both the front and rear of the enclosure. It is known that air shield systems employing a single manifold and a single vacuum source have been operated to exhaust higher air volumes in an attempt to remove additional air from behind the free-falling curtain as well as air entrained on the web.
A similar approach to minimizing the detrimental effect of boundary-layer air of the moving web is taught in U.S. Pat. No. 5,224,996 to Ghys et al. Ghys et al employed an alternative design for a curved air shield arranged in a closely spaced relationship to a backing roller which supports the moving web at the point of impingement. The alternative design for the air shield provides for increased resistance to air flow in the gap between the air shield and the backing roller at the end and side regions thereof as compared to air flow resistance at an intermediate region of the shield. A vacuum device communicates with the gap in the intermediate region to reduce air pressure therein. In such manner, there is improved removal of boundary-layer air at the surface of the moving web prior to the impingement point which apparently allows for increased speed of the moving web.
Although the prior art has dealt with minimizing the effect of boundary-layer air induced by the moving substrate, the prior art has failed to recognize or deal with the removal of entrained or boundary-layer air induced by the free-falling curtain. In fact, no negative impact on product quality in the production of photographic elements has been attributed to boundary-layer air of the free-falling curtain in the past. However, with the increased sensitivity of photographic materials achieved in recent years, product quality has become more susceptible to the detrimental effects of air currents. It has been found that on photographic products with high sensitivity, random or irregular streaks may be produced in the product even though the boundary-layer air caused by the moving web has been nullified.